In the past, a pulse wave pattern has been smoothed by smoothing the amplitude values of pulse waves with moving average. For example, see Japanese Patent Application Laid-Open (Kokai) No. Hei 5-317274.
In other words, the prior art has simply moving-averaged and smoothened time-series data of output data F (Pn) corresponding to discrete input data Pn in time series.
FIG. 4 is a characteristic curve exemplifying a relationship between differential pressures across a blood vessel wall and changes in the blood vessel volume.
It is found from such a characteristic curve that the blood volume sharply increases at a position in which the internal pressure of the blood vessel exceeds the external pressure (cuff pressure) (i.e., a sharply changing point as shown in FIG. 4). The rapid change in the volume of the blood vessel is due to the fact that the media of the blood vessel is rich in extensibility. As the differential pressure across the blood vessel wall increases, the volume of the blood vessel will not change very much. This is due to the fact that the change of the blood vessel volume is limited by the fibrous adventitia of the blood vessel.
FIG. 5 is a curve showing a relationship between the differential pressures across the blood vessel wall and the differentiated values of change in the blood vessel volume.
The prior art could not directly measure the volume of a blood vessel for measuring a blood pressure. Therefore, the prior art used a method of measuring a blood pressure value based on discrete volume changes generated by heartbeat (i.e., a pulse wave pattern) (oscillometric method).
FIG. 6 shows an actual pulse wave pattern.
The actual pulse wave (which is shown by solid line) is an uneven pattern in comparison with an ideal pattern as shown by dotted line. The uneven pattern prepared based on the actual measurements is due to the fact that the blood pressure itself fluctuates in terms of time and also due to the fact that the cuff pressure changes on movement of human's body or other factor.    Patent Publication 1 Japanese Patent Application Laid-Open (Kokai) No. Hei 5-317274.